The invention relates to a method for directing an antenna beam, the method being used in the transmission direction in a radio system which comprises transceivers forming antenna beams and in which transmitted signals, which propagate via different paths to the transceiver, are received.
It is typical to a radio network environment that a signal between a user and a base station does not propagate directly. Depending on the characteristics of the environment, the signal propagates via a plurality of paths of various lengths from the transmitter to the receiver. This kind of multipath propagation takes place, even though the base station and the mobile station were within sight of each other. This multipath propagation is mainly caused by signal reflections and scattering from surfaces in the way of the signal. Signals with different paths, which signals often arrive from different directions at the receiver, generally have different transit time delays, and consequently the signals arrive at the receiver in different phases. If a subscriber terminal, for instance a mobile telephone, is used in a moving vehicle, the reflections of the signal change as a function of time. Moreover, the relative movement of the subscriber terminal causes a change of frequency that equals to the Doppler shift with respect to the nominal frequency of the signal.
Fading occurs, if the multipath components propagating via different paths to the receiver are destructively summed at the receiver. Sometimes a carrier wave transmitted by subscriber terminals propagates fairly directly to a base station. Whereas sometimes the carrier wave reflects from several different surfaces, and consequently the carrier wave propagates to the base station via several different paths. Multipath propagation resulting from carrier wave reflections causes delay spread and angular spread in the signal to be received.
Propagation loss causes weakening in the transmitted signal, and as a consequence, the signals transmitted with the same transmission power arrive differing in strength at the receiver. Various interference signals resulting from signal reflection may be summed to the information signal in the opposite phase, whereby the information signal attenuates more and more. Various obstacles may attenuate the signal the more the further away the transmitter and the receiver are located from one another. If the signal attenuates enough, the connection between the subscriber terminal and the base station may be interrupted or the connection cannot be established in the first place.
In prior art radio systems, a base station forms an antenna beam by means of its antenna. The antenna beam consists of electromagnetic radiation emitted by the antenna. When the subscriber terminal is located in the range of the antenna beam, it is possible, in principle, to establish a connection between the base station and the subscriber terminal. The establishment of a connection is also dependent on the strength of the signal received by the subscriber terminal, for instance.
Sectored base stations are typically used in cellular radio systems.
The cell may be divided into three sectors, for instance. The base station antenna may form an antenna beam that covers the whole sector. For instance, a sector of 120 degrees can be covered by one antenna beam formed by one antenna both in the downlink and uplink directions. Thus, the coverage area needed by the downlink and uplink directions of the sector can be formed by one antenna beam. In the above-mentioned case, the subscriber terminal may be located anywhere in the cell, and nevertheless, it is possible for the subscriber terminal to establish a connection with the base station.
In prior art solutions, in which adaptive antenna arrays are applied, the signal is transmitted downlink with one narrow beam whose direction is estimated from the uplink signal. However, beam forming has been problematic at least in some operational environments. More precisely, problems arise in radio systems where the angular spread of the signal passing through the radio channel becomes large. Radio systems of the above type are, for, instance, urban micro cell systems and indoor pico cell systems.
Those signals that are used for producing radio channels in a radio system and that reflect from several different surfaces on a wide sector have a considerable amount of angular diversity. Angular diversity has been employed in uplink data transmission. The above is based on the fact that the impulse response of the uplink radio channel can be found out by estimation. Instead, the impulse response of the downlink radio channel is not known, if the changing of radio channel is observed only in the short term.
The FDD (Frequency Division Duplex) duplexing method is typically used between the downlink and the uplink, due to which the downlink and uplink radio channels do not correlate in the short term. Due to the FDD duplexing method the direction of the downlink beam, estimated from the uplink signal, may be erroneous. However, the downlink and uplink radio channels correlate, when the radio channel correlation is observed in the long term. In other words, it can be assumed that the uplink and downlink signals propagate via the same reflective surfaces (geometrical reciprocity).
Even though the same scatterers exist between the base station and the subscriber terminal, the signal, estimated by the base station, may be transmitted by a subscriber terminal that is in deep fade seen in the downlink direction. In the above-described problematic situation, the beam formed in the downlink does not necessarily cover the subscriber terminal well enough, and thus the signal was not transmitted in the best possible direction. The probability of problems with the downlink signal transmission increases, when the angular spread of the signal increases.
If the angular spread of the signal that has passed through the transmission channel is smaller than the half-power bandwidth of the base station antenna array, the downlink transmission performs in principle quite as well as the uplink transmission. In one prior art method, problems with the downlink transmission are reduced by using wider beamwidth. However, the use of the above-mentioned method leads to a new problem, i.e. to an increase in the cell size. The increased cell size requires increased transmission power, at least when only one beam is used. Moreover, the increase of transmission power adds to interference in the radio network.
The object of the invention is thus to provide a method and a transceiver to the effect that the above-mentioned problems can be solved. This is achieved with a method of the type set forth in the preamble, which method is characterized in that directions of arrival (DoA) of received signals are estimated on receiving the signals that have propagated via different paths, strengths of signals received from different DoAs are measured, and if the DoAs of the received signals differ only slightly from one another, a main antenna beam, which is directed towards the DoA of the strongest signal, is formed in the transmission direction, and if the DoAs of the received signals differ sufficiently from one another, in addition to the main antenna beam, one or more diversity beams are formed in the transmission direction, which are directed towards the DoA of the received signal, wherefrom a signal exceeding the predetermined threshold value is received.
The method of the invention is also characterized in that directions of arrival of received signals are estimated on receiving the signals that have propagated via different paths, strengths of signals received from different DoAs are measured, and the main antenna beam, which is directed towards the DoA of the strongest signal, is formed in the downlink direction of the transceiver, and one or more diversity beams are formed, which are directed towards some other direction than the main antenna beam, and the diversity beam signals are formed in such a way that the subscriber terminal serving as a receiver may differentiate the signals transmitted by different beams.
The invention also relates to a transceiver which comprises antenna arrays forming antenna beams in the transmission direction, by means of which beams a signal is transmitted, and which receives signals that propagate via different paths to the transceiver.
The transceiver in accordance with the invention is characterized by comprising estimating means for estimating DoAs of received signals while the transceiver receives the signals that have propagated via different paths, measuring means for measuring the strengths of signals received from different DoAs, and antenna beam directing means for commanding the antenna array to form in the transmission direction the main antenna beam, which is formed if the DoAs of the received signals differ only slightly from one another in the direction wherefrom the strongest signal is received, and if the DoAs of the received signals differ greatly from one another, the directing means command the antenna array to form, in addition to the main antenna beam, one or more diversity beams which are formed in the direction wherefrom a signal exceeding the predetermined threshold value is received.
The preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on the fact that angular diversity is employed in downlink transmission, and that the downlink antenna beams are directed on the basis of the received signals.
Several advantages are achieved with the method and the transceiver in accordance with the invention. By means of the method it is possible to direct the antenna beams as optimally as possible, whereby the quality of downlink transmission improves. In practice, this means that the connection signal can be transmitted with a lower transmission power than in earlier methods. If a plurality of beams are formed, the beams are directed in such a way that they do not cause interference to each other. The transceiver in accordance with the invention can be partly implemented as software, and consequently the present transceiver structure does not necessarily change much.